The present invention relates to a method of driving a display panel of a plasma display panel (referred to as PDP hereinafter) or the like, and a panel display apparatus employing the method. More particularly, the present invention relates to a method of driving a display panel in which displays with gray scale are performed by weighting each subframe to change the discharge and light emission period for display, and a panel display apparatus employing same.
Recently, in the field of display apparatus, demands for a thinner, larger-screen, and higher-resolution display apparatus are increasing information to be displayed and conditions under which display apparatuses are installed are becoming more diversified, and a display apparatus to meet these demands and cope with the diversification is required. There are several types of thin display apparatuses such as LCDS, fluorescent display tubes, ELs, PDPs, and so forth. In a fluorescent display tube, EL, or PDP, gray scale display is realized, in general, by dividing a display frame into plural subframes, weighting each subframe period, and displaying each bit of the gray scale data using a corresponding subframe. Though the following explanation will use a PDP as an example, the detailed explanation of a PDP itself is omitted here because it has been disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-160525 by the same applicants, and the gray scale representation by the subframe method and the power control relating to the present invention will be explained generally.
FIG. 1 is a block diagram showing the general structure of a general PDP display apparatus. In a panel 1, plural X electrodes and Y electrodes are arranged adjacently in turn and plural address electrodes are arranged in the direction that runs at a right angle to the aforementioned electrodes. The plural X electrodes are connected commonly, and are connected to an X driver 2. Each of the plural Y electrodes is connected to a Y driver 3. The plural address electrodes are connected to an address driver 4. A power source 5 supplies power to the X driver 2, the Y driver 3, and the address driver 4.
Though it is assumed that the input image signals are RGB digital signals, in some cases they may be analog signals and, in such a case, they are converted into digital data by an A/D converter. The input image signal is amplified by digital operations in a gain control circuit 11 and stored temporarily in a frame memory 13 by a data converter 12. At this time, the input image signal is loaded into the display plane of the frame memory 13 according to a subframe format, which will be explained later, read from each display plane according to the subframe to be displayed, and supplied to the address driver 4 as address data. The data converter 12 counts the number of lit pixels for each subframe when storing the input image signal into the frame memory 13, calculates the display load ratio, and then sends them to a driver controller 14. The display load ratio relates to the sum of light emission intensity of all cells, that is, the total number of discharge pulses for light emission (radiation pulses) in the entire panel (referred to briefly as a number of pulses in some cases hereinafter). The driver controller 14 supplies a certain gain coefficient to the gain control circuit 11, where the input image signal is multiplied by the gain coefficient. The gain coefficient is stored in a ROM or the like as a value fixed in advance, or can be set by a control that adjusts the display luminance. When the gain coefficient is set by a luminance control circuit using a variable resistor, it can be changed externally, but it is not changed automatically according to the input image signal.
A power control circuit 15 calculates the total number of pulses for a display frame based on the values of voltage and current supplied by the power source 5 and the display load ratio supplied by the driver controller 14, determines the number of radiation pulses for each subframe (SF) according to the total number of pulses, and supplies them to the driver controller 14. The period of a display frame is specified by the vertical synchronizing signal (Vsync) supplied from the outside and the signal vsync is supplied to the gain control circuit 11, the data converter 14, and the power control circuit 15, and is also supplied to the driver controller 14 via the power control circuit 15.
The driver controller 14 generates and puts out drive signals that control the X driver 2, the Y driver 3, the address driver 4, the data converter, and so forth, based on the abovementioned number of radiation pulses for each SF, Vsync, and clocks from a clock source (not shown). According to the drive signals supplied from the driver controller 14, each part generates a drive signal (waveform) to be applied to the panel 1.
FIG. 2 shows drive signals of a subframe in a PDP display apparatus of a so-called write address type, in which the address period and the sustaining discharge period are separated. The subframe will be explained later. The operations of a PDP display apparatus are explained with reference to FIG. 2. In this example, a subframe is divided into the reset period, the address period, and the sustaining discharge period. In the reset period, all cells are set to the same status. In the address period, scan pulses are applied to the Y electrodes sequentially, and address pulses are synchronously applied to the address electrodes according to the display data (address data). While scan pulses are applied to the Y electrode of a line, address pulses are applied to the address electrode of a cell that is caused to emit light among the cells of the line, and address pulses are not applied to the address electrode that is not caused to emit light. In the cell to which address pulses are applied, address discharge is caused to occur and wall-charge is accumulated on the surface of the electrode of the cell. This process is applied to all lines in succession. As a result, all cells are set to a status in accordance with the display data of the subframe, and wall-charge accumulates. In the sustaining discharge period, sustaining pulses are applied to the Y electrodes and the X electrodes alternately, and discharge is caused to occur in a cell in which wall-charge has accumulated and the cell emits light. In this case, the luminance is determined by the length of the sustaining discharge period, in other words, the number the sustaining pulses (number of times discharge for light emission is performed).
The gray scale representation in a PDP is realized by dividing a display frame into plural subframes and by combining the lit subframes. The luminance of each subframe is determined based on the number of the sustaining pulses. Though the luminance ratio of each subframe may be set at a special ratio in order to suppress the problem of color false contour, in a subframe structure as shown in FIG. 3, each term of the luminance ratio is a value of 2 to the nth power and the number of gray levels for a certain number of subframes is the maximum, therefore, this structure is widely used. In FIG. 3, the ratio of the number of sustaining discharge pulses of six subframes SF0 through SF5 is 1:2:4:8:16:32, and 64 gray levels can be represented by combining them, and each bit of the 6-bit display data corresponds to SF0 through SF5 in this order. For example, when the gray level of a display data of a cell is the 25th (1A in the hexadecimal system), SF1, SF3, and SF4 are lit and other SF0, SF2, and SF5 are not lit. Here, the total number of sustaining pulses of all the subframes of a display frame is referred to as the total number of radiation pulses n. That is, the total number of radiation pulses is the number of sustaining pluses when all the subframes are lit, and is also the number of pulses that can be lit to the maximum extent for a cell during a display frame, for example, 63 in FIG. 3.
Generally, display data supplied from the outside has a format in which the gray level of each pixel is continuous and cannot be changed to a format acceptable for the subframe directly. Therefore, the display data is stored into the frame memory 13 temporarily, read according to the format of the subframe, and then supplied to the address driver 4. In each subframe, the operation as shown in FIG. 2 is performed and only the length of the sustaining discharge period (that is, the number of sustaining pulses) differs in each subframe.
When a bright image is displayed, the number of discharge pulses for light emission of each cell increases, the total number of radiation pulses of a display frame increases, and the power consumption, that is, the current consumption increases as a result. Here, the abovementioned display load ratio is used as a quantity that relates to the brightness of the displayed image. The maximum number of radiation pulses of a display frame for the entire screen is achieved when all the cells are lit with all the radiation pulses, and the display load ratio is a ratio of the total number of radiation pulses for light emission of all cells for a display frame to the maximum number of radiation pulses. The display load ratio is 0% when all cells are displayed in black, and 100% when all cells are displayed with the maximum luminance.
In the PDP display apparatus, since the current that flows during the sustaining discharge period influences the amount of current consumption significantly, the current consumption increases when the total number of radiation pulses of a display frame increases. FIG. 4 shows the relation between the display load and the power. If the number of sustaining pulses of each subframe, that is, the total number of radiation pulses n, is fixed, the power consumption P (or current consumption) increases as the display load ratio increases.
A limit to the power consumption is specified for the PDP display apparatus. It is possible to set the total number of radiation pulses n so that the power consumption is below the limit even when the display load ratio is the maximum, that is, all cells are lit with the maximum luminance. The display load ratio is, however, around 10% to tens of percents normally, and is seldom near 100%, and a problem that the normal display is rather dark may come up. Therefore, the power control is performed by the way in which the total number of pulses n is set so that the power consumption P reaches the limit when the display load ratio is A, and when the display load ratio exceeds A, the power consumption P is controlled to be below the limit by reducing the total number of radiation pulses n. In this way, the total number of radiation pulses n decreases when the display load ratio exceeds A, and the reduction in the total number of radiation pulses n is compensated for by allocating a number of sustaining pulses to each subframe according to the specified ratio. For example, in a case where a display frame is comprised of the six subframes SF0 through SF5 as shown in FIG. 3, the ratio of the numbers of sustaining pulses is set at 1:2:4:8:16:32, and the total number of radiation pulses n (total number of sustaining pulses) is 252, the ratio of the numbers of sustaining pulses for SF0 through SF5 is 4:8:16:32:64:128 when the display load ratio is A or less, and is set at 3:6:13:26:51:103 when the display load ratio exceeds A and the total number of pulses n is reduced by 20%, down to 202.
As mentioned above, when the display load ratio increases, power control is performed by the way in which the total number of radiation pulses n is reduced to suppress the power consumption, but when the total number of radiation pulses n is rather small, a subframe-weighted distribution of sustaining pulses is not available and a problem that the number of gray levels decreases because the subframe of SF0 is not lit. As shown in FIG. 5, when the display load ratio exceeds B, the total number of radiation pulses drops below C and a decrease of a number of gray levels may be caused. A finer display with a larger number of gray levels is required, and in a case where a display frame is comprised of eight subframes and the total number of radiation pulses n is 255, the number of radiation pulse of SF0 is 1. When the total number of radiation pulses n is reduced to 127, the number of radiation pulses of SF0 becomes zero. Therefore, a problem that the number of gray levels drops may occur. In an animation display, the display lacks realism when cells with the same intensity of luminance are contiguous in a wide area, therefore, a method called error diffusion, in which noise is added intentionally, is employed. In this case, however, the diffused bits are not lit when the subframe SF0 with the least luminance is not lit, and this brings up a problem that the display lacks realism.
Here, as shown in FIG. 6, it is possible to terminate the power control when the total number of pulses reaches C, but in this case a problem that the power consumption P exceeds the limit when the display load ratio exceeds B may come up.
The object of the present invention is to realize a method of driving the display panel that provides displays with realism even when the power is controlled, and a display apparatus employing the abovementioned panel display.
To realize the abovementioned object, the power consumption is controlled by means of both the control of the total number of times discharge for light emission is performed in the plural subframes and the control of the gain of input image signals.
FIG. 7 illustrates the principle of the present invention.
Actually, in the control of power consumption as shown in FIG. 7, the total number of times discharge radiation is performed (n) is reduced first when the display load ratio increases and exceeds A, with the gain G being fixed to a constant value, and the gain G is reduced when the total number of times discharge for light emission is performed drops to the threshold C, with the total number of times discharge for light emission is performed being fixed to the threshold C. The threshold C for the total number of times discharge is performed is determined based on the minimum number of times discharge is performed, which is required to maintain the specified relation of luminance between the plural subframes. It is preferable that the threshold can be set from the outside of the display panel.
According to the present invention, power can be controlled without the occurrence of a decrease in a number of gray levels or a display of a poor quality gray scale.
When the gain is reduced, the level of the input image signal is reduced and the allocated number of gray levels is also reduced. For example, when the gain is halved for an input image signal with 100 levels, the number of levels is also halved to 50 and the luminance drops because subframes are combined to display only 50 levels.